In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These aromatic streams also contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes.
The aromatic streams are used as feedstocks in various subsequent petrochemical processes. In certain of these processes, such as para-xylene production, e.g., from an aromatic stream containing benzene, toluene and xylene (BTX) or toluene disproportionation, hydrocarbon contaminants cause undesirable side reactions. Therefore the hydrocarbon contaminants must be removed before subsequent processing of the aromatic streams.
Moreover, the shift from high-pressure semiregenerative reformers to low-pressure moving bed reformers results in a substantial increase in contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from the aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams such as BTX by contacting the aromatic stream with acid-treated clay. Other materials, e.g., zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material, while zeolites used for this purpose generally are synthesized and are therefore more expensive. Both clay and zeolites have very limited lifetimes in aromatics treatment services. The length of service correlates with the level of bromine reactive impurities (“BI-reactive” impurities or contaminants) in the feedstream. BI-reactive contaminants rapidly age both clay and zeolites. Indeed, although clay is the less expensive of the two alternatives, large aromatic plants can spend more than a million dollars a year on clay. Furthermore, since zeolites are considerably more expensive than clay, their use in removing hydrocarbon contaminants can only be justified by dramatically improved stability in aromatics treatment so that their cycle length is practical.
U.S. Pat. Nos. 6,368,496 and 6,781,023 teach bromine reactive hydrocarbon contaminants are removed from aromatic streams by first providing an aromatic feedstream having a negligible diene level. The feedstream is contacted with an acid active zeolite catalyst composition under conditions sufficient to remove mono-olefins. The aromatic stream may be pretreated to remove dienes by contacting the stream with clay, hydrogenation or hydrotreating catalyst under conditions sufficient to substantially remove dienes but not monolefins.
Other relevant references include, U.S. Pat. Nos. 6,500,996; 6,781,023 and U.S. Patent Application Publications 20060270886; 20070112240; 20070112239; 20070129235; and 20080128329.
Current clay treater reactor designs typically have a feed pre-heater capable of heating the feed to a maximum of 205° C., and operate at a maximum of 2 WHSV. Also, conventional clay treater designs do not have the capability of recycling product. The current features limit the optimization potential when using zeolites in place of clay.
The present inventor has surprisingly discovered a method of reducing the cost and reducing the environmental impact of the current process used to remove olefins from aromatics plant feedstocks, which in embodiments is achieved by operating zeolite treatment reactors similar to continuous stirred tank reactors (CSTR).